Q: We have a lovely patio in our backyard that borders a wooded area. This past fall we started noticing pinhead-sized spots (looking like tar) on our patio furniture and privacy fence. We have tried removing these spots with Bug and Tar remover, Goop and similar products to no avail.

When we use our thumbnails to scrape off the spots, the tar comes up, but it leaves a rust-like stain on the furniture/fence. We are wondering if these spots could be coming from the woods behind us.

A: I think the spots are caused by artillery fungus, also called shotgun fungus. It forms extremely small (0.1 inch) balls on decaying leaves and mulch and ejects spores in tiny globs that adhere to nearby surfaces.

As the fungal balls mature they absorb water and build up tremendous pressure inside. On a warm day the ball ruptures, sending spores up to twenty feet away. The fungus is very common but no research has been done on controlling it with fungicides.

Occasionally stirring the mulch and leaves adjacent to the fence will help keep it dry. Since the fungus discharges toward light (or light-colored surfaces), covering the area near the fence with pine straw will interfere with the projectiles.

Artillery fungus doesn’t harm plants but the spores can be unsightly on vertical surfaces. They are extremely difficult to remove; a razor scraper seems the best tool.

Cornell University has a good fact sheet.

See Artillery Fungus

How to Remove Artillery Fungus Spots